High power RF generation with optically activated bulk GaAs devices

ABSTRACT

Utilizing sections of charged transmission line cables and  optically-actied semiconductor switches, the direct generation of high power RF is demonstrated.

The invention described herein may be manufactured, used and licensed byor for the Government without payment to us of royalty thereon.

TECHNICAL FIELD

This invention relates generally to radio frequency (RF) pulse

power generation and more particularly to apparatus for generating RFpulse power from direct current (DC) by means of GaAs direct Switches.

BACKGROUND OF THE INVENTION

The conversion of DC energy directly to RF pulses with efficiency hasmany potential applications. Among such uses are: high-power electricalpulses for pulsed power devices and plasma-physic experiments; highresolution radar and time domain metrology; and the generation ofmegawatt level microwave and millimeter-wave pulses. These applicationsrequire the development of an appropriate switch or an array of switcheswhich can switch high power with extremely fast rise time and zerojitter. A frozen wave generator has been developed, consisting of manysegments of transmission line charged alternately with positive andnegative voltage. Two adjacent segments are joined by a silicon switchwhich can be closed with a laser pulse. This concept is described byChang, et al., in "Direct DC to RF Conversion By PicosecondOptoelectronic Switching," IEEE MTT-S International Microwave SymposiumDigest, May 1984 which is herein incorporated by reference. A great dealof effort has been devoted to the investigation of photo-conductivesemiconductor devices. Bulky microwave tubes and slow reset times havebeen associated with conventional RF generators. The present inventionaddresses these problems and more.

SUMMARY OF THE INVENTION

It is an object of the invention to produce an optically-activated RFgenerator with increased power output.

It is a further object of the invention to provide a RF generator havingcompact design which is capable of a fast rise time.

It is still a further object of the invention to produce a very costeffective RF generator.

The above and other objects are achieved in accordance with theinvention wherein a transmission line is comprised of an output end, anopen-terminated input end and a series of transmission line sectionsbeing serially disposed between the input end and the output end. Eachtransmission line section has a signal transmitting conductor which iselectrically isolated from the signal transmitting conductors of theadjoining transmission line sections in the series of sections and asignal return conductor which is electrically interconnected with thesignal return conductors of the adjoining transmission line sections inthe series of sections. A DC power supply means is coupled to the signaltransmitting conductors of the series of transmission line sections forcharging each of the signal transmitting conductors with DC voltagepotential standing waves having a magnitude commensurate with themagnitude of the RF pulse energy to be produced and polarities which areopposite to the polarities of the DC voltage potential standing waves towhich the adjoining signal transmitting conductors in the series oftransmission line sections are charged. A plurality ofoptically-activated GaAs normally-open switches which each have a blockof GaAs and a pair of switch electrodes mounted on opposite faces of theblock are coupled in series with a load. At least one of pair of switchelectrodes has a plurality of light-transmitting apertures formedtherein which permit light striking the electrode to reach the face ofthe block beneath the electrode. A circuit means is coupled to theswitch electrodes of the plurality of GaAs switches for coupling one ofthe switches in series circuit with a load across the output end of thetransmission line and for coupling each of the remainder of switches inseries circuit between the signal transmitting conductors of a differentone of the pairs of adjoining transmission line sections in thetransmission line. A fiber optic means is coupled between a laser meansand the light apertured electrodes of all of the plurality of GaAsswitches. The laser means emits pulses of light at a predeterminedrepetition rate, the light pulses being at a wavelength which will causethe blocks of GaAs in each of the plurality of switches to close and tothereby connect the signal transmitting conductor of the transmissionline in series circuit with the load, whereby a RF pulse transmission ofthe DC voltage potential standing waves is transmitted to the load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a three-switch RF generator.

FIG. 2 displays a waveform from the RF generator.

FIG. 3 illustrates a gridded optically activated switch (OAS) device.

FIG. 4 is the graphic response of a laser light pulse leaving the fiberoptic bundle.

FIG. 5 displays a switch current waveform.

FIG. 6 illustrates a RF waveform obtained by a current transformer.

DETAILED DESCRIPTION OF THE DRAWINGS

The RF generator consists of charged coaxial transmission lines (PFL)and gridded GaAs OAS devices. As shown in FIG. 1, the circuit has threesegments of PFL's charged by positive and negative voltage Vo. AdjacentPFL's are connected with a bulk GaAs OAS. Since semi-insulating GaAs hasa high resistivity (>10⁷ ohm-cm), the leakage current is negligible.When all switches are fired simultaneously, the standing waves, whichhave amplitude Vo/2, start to move in the forward and backwarddirections. The forward wave travels toward the output load and appearson the load resistor. The backward wave moves toward the opentermination end, is totally reflected from the open termination end, andthen moves back toward the output load. The half period of the RFgenerated is the transit time for the standing wave to travel across thePFL of length L. The repetition frequency of the burst of RF isdetermined by the repetition rate of laser system. The predictedgeneration of the RF burst is given in FIG. 2.

In most high power bulk OAS devices the light is introducedperpendicular to the applied field direction. Although these devicesshowed very high power capability, multiple OAS operation requiredhighly efficient devices as well. Recently, optically activated griddedsilicon PIN diodes demonstrated high efficiency. Utilizing this concept,optically activated gridded bulk GaAs devices were designed andfabricated at ETDL. The typical bulk GaAs OAS is shown in FIG. 3. Inthese switches the light is introduced parallel to the applied fielddirection. The test results of these devices shows significantimprovement of efficiency. With light from a 20 ns Q-switched Nd:YAGlaser, emanating a fiber optic bundle, the switch turn-on was sustainedfor 150 ns without distortion. The FIG. 4 shows the laser light pulsewaveform leaving the fiber optic bundle. FIG. 5 shows the resultingswitch current waveform with a 200 ns PFL, biased at 2 KV. The OAS wasturned on with laser energy as small as 0.8 mJ. The current latch-on andlow threshold laser energy is assumed due to field-introduced avalancheeffect. On-state voltage was very small. The parametric relationship ofapplied voltage, optical energy, and output pulse width will bediscussed elsewhere.

It is necessary to supply each of the switches with sufficient laserenergy. This was implemented with fiber optic bundles. The advantage ofthis technique is that each bundle may contain varying fiber lengths,thus effectively increasing the width of the light pulse and insuringthe switch stays on, until all forward and backward waves pass through.

This is particularly critical for switch number 3, which must stay onthe longest time In this experiment, however, it was unnecessary to usevarying fiber lengths, since the OAS gridded device stayed on fairlylong, up to 150 ns. In order to achieve wider burst pulse widths,however, the use of incremental fiber lengths may be expected to play arole.

Each switch was tested individually at 2 KV DC bias with 0.8 mJ lightenergy from a Q-switched Nd:YAG laser (20 ns pulse width). The coaxialcable lengths of the 3-stage generator were 9 feet, 9 feet, and 4.5 feetrespectively, with an anticipated total pulsewidth of 67.5 ns. Sincethis width is less than the recovery time (150 ns) of the OAS,simultaneous triggering of the switches was employed, and thus equallengths of fibers were used to convey the light to the switches. Atypical waveform, is shown in FIG. 6, obtained by a current transformer,the Tektronix CT-1.

The maximum current achieved was approximately 12 amps, with apulsewidth of about 100 ns, instead of the anticipated values of 20 ampsand 67.5 ns. The longer pulsewidth, as well as the lower currentamplitude, are caused primarily by inductance in the connections betweenOAS devices. This inductance will be eliminated by incorporating theswitch into the transmission line. Narrower light pulses also will beused to minimize the effect of the risetime of the light signal. Unequaldistribution of the light among the various switches must also beaddressed.

What is claimed is:
 1. An optically-activated RF generator for producinghigh power RF pulse energy from applied DC energy comprising:atransmission line having an output end, an open-terminated input end anda series of transmission line sections serially disposed between saidinput end and said output end, each of said transmission line sectionshaving a signal transmitting conductor which is electrically isolatedfrom the signal transmitting conductors of the adjoining transmissionline sections in said series of sections and a signal return conductorwhich is electrically interconnected with the signal return conductorsof the adjoining transmission lines sections in said series of sections;a DC power supply means coupled to the signal transmitting conductors ofsaid series of transmission line sections for charging each of saidsignal transmitting conductors with DC voltage potential standing waveshaving a magnitude commensurate with the magnitude of the RF pulseenergy to be produced and polarities which are opposite to thepolarities of the DC voltage potential standing waves to which theadjoining signal transmitting conductors in said series of transmissionline sections are charged; a load; a plurality of optically-activatedGaAs normally-open switches, each of said switches having a block ofGaAs and a pair of switch electrodes mounted on opposite faces of saidblock, at least one of said pair of switch electrodes having a pluralityof light-transmitting apertures formed therein which permit lightstriking the electrode to reach the face of the block beneath theelectrode; a circuit means coupled to the switch electrodes of saidplurality of GaAs switches for coupling one of said switches in seriescircuit with said load across the output end of said transmission lineand for coupling each of the remainder of said switches in seriescircuit between the signal transmitting conductors of a different one ofthe pairs of adjoining transmission line sections in said transmissionline; a laser means for emitting pulses of light at a predeterminedrepetition rate, said light pulses being at a wavelength which willcause the blocks of GaAs in each of said plurality of switches to becomeelectrically conductive; and a fiber optic means coupled between saidlaser means and the light apertured electrodes of all of said pluralityof GaAs switches for causing all of said switches to close and tothereby connect the signal transmitting conductor of said transmissionline in series circuit with said load, whereby a RF pulse transmissionof said DC voltage potential standing waves is transmitted to said load.